Vaccines to protect against viral infections have been effectively used to reduce the incidence of human disease. One of the most successful technologies for viral vaccines is to immunize animals or humans with a weakened or attenuated strain of the virus (a “live, attenuated virus”). Due to limited replication after immunization, the attenuated strain does not cause disease. However, the limited viral replication is sufficient to express the full repertoire of viral antigens and generates potent and long-lasting immune responses to the virus. Thus, upon subsequent exposure to a pathogenic strain of the virus, the immunized individual is protected from disease. These live, attenuated viral vaccines are among the most successful vaccines used in public health.
Yersinia is a genus of bacteria in the family of Enterobacteriaceae. Yersinia are facultative anaerobes. Some members of Yersinia are pathogenic in humans. Often, rodents are the natural reservoirs of Yersinia; less frequently other mammals may serve as a host to these bacteria. Infection can occur either through arthopod bite, exposure to blood, aerosol transmission (e.g. Y. pestis), or by, for example, consumption of food products (e.g. vegetables, milk-derived products and meat) contaminated with the bacteria. Other modes likely exist (e.g. via protozoonitic mechanisms) for transmission. The Yersinia family is rather large, but only two have been linked to water-borne outbreaks of disease, Y. pseudotuberculosis and Y. enterocolitica. Yersinia species are found all over the world in animal reservoirs (e.g., rodent reservoirs for Y. pestis), isolated in well-water, water treatment plants, rivers and lakes. Yersinia pestis (also referred to as Pasteurella pestis) is the most famous member of the Yersinia species and is the causative organism of plague.